The present invention relates to a vehicle steering system.
Vehicle steering systems suitable for xe2x80x9csteer-by-wirexe2x80x9d operation have been described. In such systems, a driver can enter a steering command into the vehicle steering system from the steering grip, usually a steering wheel, by exerting a manual force on the steering wheel, thus causing the steering wheel to turn. A steering angle setpoint generator determines that the steering wheel has been turned and communicates this information to a controller arrangement of the vehicle steering system. The controller arrangement also interrogates the steering angle currently set at steerable vehicle wheels using a steering angle actual-value sensor and carries out a setpoint/actual-value comparison of the steering angle. From this comparison, the controller arrangement derives a control command for an actuating unit, which operates the steerable vehicle wheels accordingly.
A steering system of this type also has a manual force regulator that simulates manual steering forces or steering forces on the steering wheel. Steering forces of this type can be, for example, a resistance counteracting the manual steering force and/or lateral forces acting upon the steerable vehicle wheels. A manual force regulator of this type is used primarily to improve vehicle response and thus helps improve control over the vehicle.
Vehicle steering systems of this type do not need a conventional drive link between the steerable vehicle wheels and the steering wheel. However, a mechanical or hydraulic drive of this type is frequently also provided and can be activated upon reaching an xe2x80x9cemergency levelxe2x80x9d, thus creating a redundant steering system.
Because the steering grip in a vehicle steering system of this type is flexibly coupled with the manual force regulator via a flexible coupling element, in particular via a spring device, the arrangement composed of the manual force regulator, coupling element, and steering grip forms an oscillating system. Vibrations occurring in the vehicle or in the steering system can therefore be transmitted to this arrangement and produce relative motions between the manual force regulator and steering grip. The relative motions produced in this manner can, in turn, lead to changes in the values at the setpoint generator, thus producing unwanted control or steering motion at the steered vehicle wheels.
Vibrations, in particular torsional vibrations of the steering grip, can also produce unwanted driving angle variations or driving angle vibrations, for example if the driver releases the steering wheel while it is in a steering state in which the manual force regulator simulates a manual force. In this case, the flexibility of the coupling element produces a steering countermotion which the manual force regulator cannot counteract quickly enough. The delayed resistive force of the manual force regulator can then support the return vibration of the steering wheel which has already begun. Disadvantageously, these vibrations and counter-vibrations can escalate. During steering wheel movement, new steering angle setpoints are constantly being generated which produce corresponding control events and steering movement at the steered wheels.
An object of the present invention is to provide a vehicle steering system which reduces the formation of vibrations between the steering grip and the manual force regulator.
The present invention provides a vehicle steering system having a steering grip (9), in particular a steering wheel, which is operated by the driver; an actuating unit (4) which operates steered vehicle wheels (1); a steering-angle setpoint generator (10) which is operated by the steering grip (9); a steering-angle actual-value sensor (21) which is operated by the steered vehicle wheels (1); a controller arrangement (19) which activates the actuating unit (4) as a function of a setpoint/actual-value comparison of the steering angle; and a manual force regulator (12) which is coupled via a flexible coupling element (11) with the steering grip (9) and simulates parameter-dependent steering forces on the latter, characterized in that a damping device (27) are provided which interact with the steering grip (9) either directly or indirectly and damp relative adjusting motions between the steering grip (9) and the manual force regulator (12).
The present invention is based on the general idea of damping the relative motions between the steering grip and the manual force regulator, in particular the movements of the steering grip, with the help of a correspondingly arranged damping device. This action reduces the tendency of the oscillating system formed by the manual force regulator, flexible coupling element, and steering grip to vibrate.
Because the steering angle setpoint generator is usually coupled with the steering grip, damping the movements of the steering grip causes unwanted movements of the steering grip to produce only minor changes, or none at all, in the setpoint steering angle, thus producing correspondingly minor changes in the actual steering angle. This reduces the danger of vibrations in the steering system.
According to one advantageous embodiment of a vehicle steering system according to the present invention, the flexible coupling element can be designed in the form of a shaft composed of a first and a second shaft section. The one shaft section is rigidly connected to the manual force regulator, while the other shaft section is rigidly connected to the steering grip. Both shaft sections are coupled with one another by a spring device, such as a C-spring or a torsion bar or torque rod, thus allowing them to rotate relative to one another against the elastic resistance. In an embodiment of this type, the damping device is arranged so that it damps the rotations of the shaft sections relative to one another.
Further important features and advantages of a vehicle steering system according to the present invention may be understood from the description below.
The features described above, and those still to be explained below can, of course, be used not only in the combinations mentioned but also in other combinations or even by themselves without exceeding the scope of the present invention.